Prior art flame-retardant resin compositions used for the insulation and the sheath of electric wires and cables include compositions such as those produced by mixing a flame retardant such as antimony trioxide, a chlorine flame retardant or a bromine flame retardant with a polyethylene, ethylene-vinyl acetate copolymer, ethylenepropylene rubber, polyvinylchloride resin, chloroprene rubber, chloro sulfonated polyethylene rubber or the like.
Because these compositions contain halogens in their base resins or flame-retardants, they produce at high temperatures large amounts of toxic gases such as a hydrogen halide gas and a halogen gas, resulting in a serious health and safety problem. Heavy smoke emission that results from the combustion is another problem. Thus, products using these compositions are not appropriate to use in a place where safety is highly required, for example, an underground railway, building, ship, nuclear power plant. Furthermore, halogen gases and hydrogen halide gases are not advantageous because they corrode the adjacent conductor and the like.
It is known to use inorganic fire retardants such as metal hydroxides rather than halogen containing compounds to produce flame-retardant polymer compositions; however, rubber or plastic materials free of tiny halogen are inferior in flame retardancy to compositions using materials containing a halogen and a halogen flame retardant. The high levels of metal hydroxides required to achieve adequate fire retardance undesirably decreases mechanical and electrical insulation properties of the polymer. In particular, compositions with high levels of metal hydroxides exhibit reduced mechanical characteristics especially in respect of tensile strength and elongation and impaired aging characteristics. Various methods have been used to restore and improve properties, such as careful choice of polymer and copolymer compositions, use of selected "coupling agents," and cross-linking during or after melt forming the material on such forms as wire coatings.
With respect to the conventional cable, when the inner structural members of the cable core made of materials such as polyethylene and cross-linked polyethylene are exposed to flames in a fire, they become molten and flow to the surface of the cable where they are gasified in a high temperature atmosphere and immediately catch fire, so that the cable burns and makes the fire larger.
In order to improve the conventional flame-retardant resin compositions and thermal resistance in addition to flame retardancy, cross-linking is made on the base resins such as polyethylene and ethylenepropylene rubber. Cross-linking is customarily performed after the polymer composition has been applied to the electric wire or cable, because cross-linked polymers are generally intractable and not processible. Chemical cross-linking and electron beam cross-linking are frequently used for such treatment. In chemical cross-linking the composition is usually heat treated by steam, etc., under a high temperature and pressure. Electron beam cross-linking requires an electron beam emitting chamber and apparatus.
In conventional chemical cross-linking of the polymer which has been applied to the electric wire or cable, by the use of a peroxide or the like, a material adjacent to the polymer composition to be cross-linked can be deformed because the cross-linking is carried out under a high temperature and pressure. For example, the inner core of the cable can be heat deformed by the cross-linking of the sheath thereof. On the other hand, in electron beam cross-linking it is difficult for an electron beam to reach to the innermost of the polymer composition layer to be cross-linked resulting in nonuniform cross-linking and poor mechanical properties. For example, in a relatively thick sheath only the surface portion thereof can be cross-linked. Electron beam cross-linking is disadvantageous in that it is particularly difficult to completely cross-link a thick layer of the composition. In addition, chemical cross-linking or electron beam cross-linking often require large cross-linking equipment which increases equipment and maintenance costs.
U.S. Pat. No. 4,549,04 1 describes a cross-linked composition which may contain an olefin resin including an olefin copolymer, metal hydroxides and coupling agents, and which incorporates a vinyl silane grafted olefin resin which provides a site which is cross-linkable by moisture with or without a "catalyst" after and possibly during extrusion forming of product such as coated wire. The finished products such as wires and cables remain intact and have the improved properties which cross-linking can achieve; however, the process is cumbersome and expensive because components of the complete composition must be partially preblended and kept dry and separate to prevent cross-linking prior to a final blending and extrusion forming step. There is no indication that cross-linking prior to extrusion forming would be desirable, practical, or produce a composition which was processible.
U.S. Pat. No. 4,769,179 describes a composition which may contain an ethylene copolymer and optionally polyethylene, a hydrate of a metallic oxide and a phosphorus containing titanate coupling agent. The composition is cross-linked by use of 0.1 to 20 parts of a cross-linking agent or ionizable radiation. There is no indication that cross-linking is controlled and that cross-linked compositions could be formed into molded or extruded forms such as coated wire.
U.S. Pat. No. 4,839,412 discloses a composition containing ethylene copolymers, optionally polyethylene, aluminum or magnesium hydroxide together with maleic anhydride grafted polymer. The combination attempts to provide a balance of mechanical properties and fire retardancy. The composition is not cross-linked.
None of the prior art references disclose a flame-retardant composition which is partially cross-linked before melt forming and retains good mechanical properties and flame retardancy.